1. Field of the Invention
The present invention relates generally to transmit diversity in an NB-TDD CDMA (Narrow Band Time Division Duplexing Code Division Multiple Access) mobile communication system, and in particular, to an apparatus and method for determining whether transmit diversity is used by a signal transmitted over a primary common control physical channel (P-CCPCH), a downlink channel.
2. Description of the Related Art
In general, “transmit diversity” or “transmission diversity” refers to a scheme for transmitting the same information through a plurality of antennas for accurate transmission of the information. The transmit diversity is chiefly used for a channel transmitting important information that should not be transmitted incorrectly.
FIG. 1 illustrates a multiframe structure used in an NB-TDD CDMA mobile communication system. Referring to FIG. 1, a multiframe is comprised of a plurality of frames, and information by P-CCPCH is transmitted over only first and second frames of the multiframe. The information transmitted by the P-CCPCH includes BCH (Broadcast Channel) information. That is, the BCH information is transmitted over the P-CCPCH in a period where a data symbol with a downlink time slot #0 of each subframe is transmitted. The structure of the multiframe is represented by reference numeral 101. Reference numeral 102, i.e. a radio frame, shows a structure of the first frame among a plurality of the frames constituting the multiframe 101. The second frame for transmitting information through the P-CCPCH also has the same structure as shown by the reference numeral 102. Each frame shown by the reference numeral 102 has a length of 10 ms, and is comprised of two subframes.
Reference numeral 103 shows a structure of one subframe of the two subframes contained in radio frame 102. Each subframe 103 has a length of 5 ms and includes 7 time slots Ts0-Ts6. The respective time slots are used for uplink transmission or downlink transmission. Each time slot is available for unidirectional transmission, i.e., for either uplink transmission or downlink transmission. How many time slots out of the 7 time slots in one subframe are to be used for the uplink transmission or downlink transmission, can be arbitrarily set by the system operator. Between the first time slot and the second time slot, there exist a 96-chip downlink pilot time slot (DwPTS), a 96-chip guard period (GP) and a 160-chip uplink pilot time slot (UpPTS). The DwPTS is used for initial cell search, random access, synchronization or channel estimation at a UE (User Equipment). The UpPTS is used for uplink synchronization and channel estimation at a UTRAN (UMTS (Universal Mobile Telecommunication System) Terrestrial Radio Access Network).
Reference numeral 104 shows a detailed structure of the first time slot Ts0 and the DwPTS, shown by subframe 103. Each time slot has a total length of 864 chips (=1,675 μs) as shown by the reference numeral 104. The time slot is comprised of two 352-chip data symbol periods, a 144-chip midamble period intervening therebetween, and a 16-chip guard period (GP). For the time slots transmitted from the UTRAN over the downlink, the midamble signal is used by the UE to determine which channels are transmitted from the UTRAN and to estimate a channel environment between the UE and the UTRAN. In addition, for the time slots transmitted from the UE over the uplink, the midamble signal is used by the UTRAN to determine which UE is transmitting the channel and to estimate a channel environment between the UE and the UTRAN. The midamble signal is associated with a specific uplink/downlink channel, and can be used in determining which channel or which subscriber transmits the channel. The GP has a 16-chip length, and serves to separate the time slots.
The DwPTS, as shown by the reference numeral 104, is comprised of a total of 96 chips. The 96-chip DwPTS is divided into a 32-chip GP and a 64-chip synchronization (SYNC) code. The SYNC code is used not only to acquire synchronization but also to indicate information on the first time slot in one subframe.
The NB-TDD CDMA mobile communication system transmits a broadcast channel (BCH), a transport channel, through the P-CCPCH. The BCH information is necessary for UTRAN synchronization and UTRAN information acquisition process.
FIG. 2 illustrates a cell search process, the UTRAN synchronization and UTRAN information acquisition process. As illustrated in FIG. 2, the cell search process can be divided into 4 steps: a first step of receiving information on a serving UTRAN to which the UE currently belongs, a second step of identifying a scrambling code and a basic midamble code in use, a third step of acquiring multiframe synchronization, and a fourth step of accessing information transmitted over the broadcast channel.
Referring to FIG. 2, in DwPTS search step (Step 201), the UE searches for signals received at the DwPTS for a SYNC code using a matched filter. There exist a total of 32 kinds of the SYNC codes, and the SYNC codes are used for synchronization between the UE and the UTRAN. Since each UTRAN uses a specific one of the 32 SYNC codes, the UE should identify the SYNC code used by the serving UTRAN to which it currently belongs.
After recognizing the SYNC code used by the serving UTRAN in Step 201, the UE performs a step of identifying a scrambling code mapped with a basic midamble code on a one-to-one basis. Step 202 of FIG. 2 corresponds to a step of identifying the scrambling code and the basic midamble code. One SYNC code is associated with four basic midamble codes, and since the UE continuously receives the midamble signals from the synchronized UTRAN, the UE identifies one of the four basic midamble codes, which is matched to the UTRAN. The “midamble signals” refer to codes obtained by separating a 256-chip code determined by consecutively combining two 128-chip basic midamble codes, in a unit of 144-chip length at stated periods. Therefore, by continuously receiving the midamble signals, the UE determines which midamble code is matched to the UTRAN. By recognizing the basic midamble code, the UE identifies the scrambling code used by the UTRAN. This is because the basic midamble code is mapped with the scrambling code on a one-to-one basis.
After searching for the SYNC code, the basic midamble code and the scrambling code used by the serving UTRAN in Steps 201 and 202, the UE performs a multiframe synchronization step (Step 203). In this step, the UE acquires synchronization of the multiframe comprised of 16 frames (or 32 subframes). That is, the UE searches for the head and the tail of the multiframe. In Step 203, the UE finds out a frame position where the P-CCPCH signal is transmitted according to a pattern of respective phase demodulation values, by performing QPSK (Quadrature Phase Shift Keying) phase demodulation on the signal received at the DwPTS. In this manner, the UE is informed as to which is the first frame of the multiframe. Since the P-CCPCH is transmitted over the first and second frames of the multiframe, the UE can acquire multiframe synchronization by finding out the position of the P-CCPCH signal. The signals received at the DwPTSs of the respective frames constituting one multiframe are QPSK phase modulated in order, so that the UE can search for the head (start) and the tail (end) of the multiframe.
In Step 204, the UE reads BCH information. Through Step 201 and 202, the synchronization between the UE and the UTRAN is acquired and the UE is acquainted with the scrambling code used by the UTRAN. After step 203, the multiframe synchronization, the UE acquires information on the UTRAN by receiving the P-CCPCH signal included in the respective first time slots of the first and second frames of the multiframe, completing the cell search process.
As stated above, the existing NB-TDD CDMA mobile communication system does not yet consider using transmit diversity for the signal transmitted over the P-CCPCH. This is because a downlink dedicated physical channel (DL-DPCH) is generally transmitted to the respective UEs through beam forming, so that intra-cell interference is relatively low. Therefore, the system is based on the fact that the common channels are not required to use the transmit diversity.
However, when interference increases in a congested area such as a downtown or in a situation where inter-cell interference is inevitable, a transmission gain of the common channels such as the P-CCPCH should be increased according to circumstances. Therefore, when the P-CCPCH does not use the transmit diversity, it is not possible to obtain a high transmission gain. Thus, in order to easily increase the transmission gain, it is preferable to use transmission diversity. However, up to date, no definition has been made on such a method.